[css-position-4] Restore the older Fullscreen spec's behavior for 'add to top layer'.

Author: tabatkins

css-position-4/Overview.bs

@@ -461,18 +461,11 @@ Top Layer Manipulation {#top-manip}
 	given a {{Document}} |doc|
 	and an {{Element}} |el|:
 
-	1. If |doc|'s [=Document/top layer=] [=set/contains=] |el| already:
-		1. If |doc|'s [=pending top layer removals=] [=set/contains=] |el| as well,
-			[=set/remove=] |el| from both |doc|'s [=Document/top layer=]
-			and [=pending top layer removals=],
-			then [=set/append=] |el| to |doc|'s [=Document/top layer=].
-			Return.
 
-			<!-- In other words, eagerly process pending removals. -->
+	1. [=set/Remove=] |el| from |doc|'s [=Document/top layer=],
+		then [=set/append=] |el| to |doc|'s [=Document/top layer=].
 
-		2. Otherwise, do nothing and return.
-
-	2. [=set/Append=] |el| to |doc|'s [=Document/top layer=].
+	2. [=set/Remove=] |el| from |doc|'s [=pending top layer removals=].
 
 	3. At the UA !important [=cascade origin=],
 		add a rule targeting |el|